The present invention pertains generally to methods for injecting a fluid medicament into a patient. More particularly, the present invention pertains to methods for using needleless (jet) injectors for delivering a fluid medicament to a patient. The present invention is particularly, but not exclusively, useful as a method for generating injection pressures with the fluid medicament that will effectively create a hole in the skin of the patient and subsequently maintain a substantially constant infusion pressure.
Needleless injectors have been used for many years for the purpose of infusing fluid medicaments into a patient. Indeed, they have several advantages over needle-type injectors. For instance, needleless injectors lend themselves to schedules where a large number of patients are to be inoculated at the same time. Most importantly, they do not incorporate sharp or pointed projections that can inadvertently stick into the care giver, or into some other third person. In recent years, the avoidance of so-called xe2x80x9csharps,xe2x80x9d that can cause inadvertent sticks, has been a design objective of many medical devices.
In their operation, all needleless (jet) injectors rely on the generation of fluid pressures in the fluid medicament. Specifically, the purpose of generating these pressures is two-fold. First, it is necessary to create a hole in the skin of the patient. Second, it is necessary to thereafter maintain a substantially constant pressure for infusion of the fluid medicament into the patient through the hole. The magnitude and duration of these fluid pressures will, in large part, depend on the type of injection to be given.
There are basically three different types of injections that may need to be performed by a needleless injector. These are: 1) shallow, intra-dermal injections where the fluid medicament is infused directly into the skin; 2) medium depth, subcutaneous injections where the fluid medicament is infused into the fatty tissue beneath the skin; and 3) deeper intra-muscular injections where the fluid medicament is delivered directly into muscle tissue. Thus, depending on the type of injection that is desired, and the general nature or condition of the patient""s skin, the fluid pressure that is necessary to make an appropriate hole can vary from injection to injection.
The mechanics of making an appropriate hole into or through the skin of a patient can be considered in terms of the forces that are generated for this purpose. An important aspect of this consideration involves the impulse-momentum relationships that are created to make an appropriate hole in the skin of a patient. By definition, an impulse force is one that is generated when two bodies collide with each other. In such a collision there is a large reactive force between the bodies that continues over the period of impact. This force (i.e. impulse) can be measured only by its time integral, and is equal to the change of momentum produced in either body. Further, momentum is a dynamic quantity that is conserved within a closed system and which is equal to xe2x80x9cmvxe2x80x9d where xe2x80x9cmxe2x80x9d is the mass of the body, and xe2x80x9cvxe2x80x9d is its velocity. Stated differently, an impulse force can be thought of as being equal to a change in momentum that is equal to the mass of a body times its change in velocity.
Insofar as needleless injectors are concerned, the initial immediate rise of pressure in the fluid medicament that is necessary to create a hole in the skin of a patient is typically generated by driving a drive bar against a syringe plunger. The resultant impulse force then contributes to a pressure rise in the fluid medicament. This, in turn, causes the fluid medicament to penetrate the skin, and thereby create the necessary hole for subsequent infusion. Larger and faster drive bars, of course, would increase the fluid pressure. An example of such a device is provided in U.S. Pat. No. 5,911,703 for an invention of Slate et al. that is entitled xe2x80x9cTwo-Stage Fluid Medicament Jet Injectorxe2x80x9d and that is assigned to the same assignee as the present invention.
Heretofore, when using needleless injectors, the practice has been to position a pre-filled injection tube directly against the skin of the patient. This, however, also places the fluid medicament that is in the tube in direct contact with the skin. Consequently, because the fluid medicament is already in contact with the skin, the impulse force that is created as the drive bar impacts the fluid medicament is significantly attenuated by the time its effect is felt between the fluid medicament and the skin of the patient. An initial consequence of this is that the fluid medicament has insufficient momentum to penetrate the skin. Thus, it happens, at least initially, that the fluid medicament can seep around the injector and puddle on the surface of the skin. It would be desirable, however, to effectively generate increased fluid pressures (fluid momentum) for the purpose of creating a hole in the skin of a patient. Preferably, this can be done without necessarily resorting to larger and faster drive bars, while also avoiding the seepage and puddling of the fluid medicament on the skin of the patient.
With needleless injectors there is always the requirement that a jet pressure be developed which is sufficient to cause the fluid medicament to penetrate the skin. One way to accomplish this is to use a heavy drive bar that will generate the necessary momentum. Heavy drive bars, however, also generate an undesirable recoil and, for spring-loaded injector mechanisms, a heavy drive bar will require a spring with a relatively large spring constant. Consequently, depending on the type of mechanism that is used to propel the drive bar, injector mechanisms that use large springs to propel heavy drive bars can be hard to cock. Although lighter drive bars will overcome these less desirable consequences, lighter drive bars will necessarily have less momentum under the same circumstances.
In light of the above, it is an object of the present invention to provide a method for injecting a fluid medicament into a patient that uses a gas pocket in the injection tube of a needleless injector to increase the impulse force of a fluid medicament as it is ejected from the injector for the purpose of creating a hole in the skin of a patient. It is another object of the present invention to provide a method for using a needleless injector to inject a fluid medicament into a patient that is predictable, reliable, controllable, and repeatable. Still another object of the present invention is to provide a method for injecting a fluid medicament into a patient that allows for the generation of greater fluid pressures, while using smaller injection mechanisms. Another object of the present invention is to provide a method for injecting a fluid medicament into a patient that avoids the leakage or seepage of fluid medicament onto the skin of a patient. Still another object of the present invention is to provide a method for injecting a fluid medicament into a patient that minimizes recoil and allows the injector mechanism to be easily cocked. Yet another object of the present invention is to provide a method for injecting a fluid medicament into a patient that is easy to practice, simple to employ and is very cost effective to implement.
In accordance with the present invention, a method for injecting a fluid medicament into a patient uses a typical syringe mechanism. Specifically, the syringe will have a fluid chamber and a plunger that can be advanced into, and withdrawn from, the syringe chamber. In its normal operation, as the plunger is advanced into the chamber of the syringe, the fluid medicament will be expelled from the chamber and through an injection tube. As envisioned for the present invention, this injection tube will extend from the chamber and terminate at a distal port. Preferably, between the fluid chamber of the syringe and the distal port, the injection tube is formed as a generally cone-shaped funnel. Specifically, the funnel of the injection tube will have a taper that is of a generally decreasing radius in the distal direction.
The steps for the method of the present invention include initially filling the syringe chamber with a fluid medicament. Then, importantly, a gas pocket is created in the distal portion of the injection tube that is contiguous to the port. Depending of the type of injection to be made (i.e. intra-dermal, subcutaneous, or intra-muscular), and the patient""s skin type, the volume of this gas pocket can be varied. In general, the volume of the gas pocket will be in the range of from about one to about twenty microliters. Once the gas pocket has been created, the port of the injection tube can then be positioned against the skin of the patient.
With the port of the injection tube positioned against the skin of the patient, the needleless injector is activated to force the fluid medicament through the port to inject the fluid medicament into the patient. This action can actually be considered as a two-step process. In the first step, the fluid medicament is accelerated through the gas pocket that was initially established in the distal portion of the injection tube. During its accelerated transit of the fluid medicament through the empty distal portion of the injection tube, the fluid medicament gains momentum. Then, on its impact with the skin, an impulse force is generated between the fluid medicament and the skin that is manifested as an initial pressure spike. It is this initial pressure spike that creates the hole in the skin of the patient. Thereafter, in the second step, a substantially constant infusion pressure is maintained for injecting the fluid medicament through the hole into the patient. It happens that the pressure for initially creating the hole will be greater than the subsequent infusion pressure.
Some additional considerations for the present invention are that the initial jet pressure, which creates the hole in the skin of the patient, is generally in a range between about twenty percent (20%) and about five hundred percent (500%) above the initial jet pressures generated when no gas pocket is present in the injection tube. Further, it is to be appreciated that the gas pocket in the injection tube can be created merely by withdrawing the plunger from the chamber. Other methods, of course, can be used to create the gas pocket. For instance, air can be forced into the tip, or fluid can be forced from the tip by shaking or wicking. As envisioned for the methods of the present invention, the gas pocket will preferably have a fluid volume that is in a range between one and about twenty microliters (1-20 xcexcl).